As a heater, for example, a ceramic heater disclosed in Japanese Unexamined Patent Publication JP-A 2007-240080 (referred to as Patent Literature 1, hereinafter) is known. The ceramic heater disclosed in Patent Document 1 includes: a substrate having a bar shape and formed of ceramics; and a heat-generating element buried in the substrate. The heat-generating element includes a pair of bar-shaped electrically conductive parts extending in an axial direction thereof. Then, the conductive part has a circular shape when viewed in a cross section perpendicular to the axial direction.
In recent years, a heater capable of more rapidly raising the temperature is required. In order to rapidly raise the temperature of the heater, a high current need be supplied to the heat-generating element of the heater. Nevertheless, when a high current is supplied to the heat-generating element, a possibility arises that heat generation occurs locally in a part of the heat-generating element so that a large thermal expansion occurs in that part of the heat-generating element. Then, a possibility is caused that the large thermal expansion having occurred in the heat-generating element generates cracks between the heat-generating element and the ceramic-made substrate. In particular, like in the ceramic heater disclosed in Patent Literature 1, when the cross section of the heat-generating element has a circular shape, generated cracks easily propagate along the interface between the heat-generating element and the substrate and hence a possibility is caused that the cracks grow in the circumferential direction. As a result, gaps are generates between the heat-generating element and the substrate so that the heat generated in the heat-generating element becomes difficult to be transmitted to the substrate and hence a possibility of degradation of the long term reliability of the heater is caused.